.TH std::for_each 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::for_each \- std::for_each

.SH Synopsis
   Defined in header <algorithm>
   template< class InputIt, class UnaryFunc >
   UnaryFunc for_each( InputIt first, InputIt last,         \fB(1)\fP (constexpr since C++20)
   UnaryFunc f );
   template< class ExecutionPolicy, class ForwardIt, class
   UnaryFunc >

   void for_each( ExecutionPolicy&& policy,                 \fB(2)\fP \fI(since C++17)\fP

                  ForwardIt first, ForwardIt last,
   UnaryFunc f );

   Applies the given function object f to the result of dereferencing every iterator in
   the range [first, last). If f returns a result, the result is ignored.

   1) f is applied in order starting from first.

   If UnaryFunc is not MoveConstructible, the behavior is undefined. \fI(since C++11)\fP

   2) f might not be applied in order. The algorithm is executed according to policy.
   This overload participates in overload resolution only if

   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.        (until
                                                                             C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                                                                             C++20)

   If UnaryFunc is not CopyConstructible, the behavior is undefined.

   If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of
   the range through the dereferenced iterator.

   Unlike the rest of the parallel algorithms, for_each is not allowed to make copies
   of the elements in the sequence even if they are TriviallyCopyable.

.SH Parameters

   first, last - the range to apply the function to
   policy      - the execution policy to use. See execution policy for details.
                 function object, to be applied to the result of dereferencing every
                 iterator in the range [first, last)

                 The signature of the function should be equivalent to the following:

   f           -  void fun(const Type &a);

                 The signature does not need to have const &.
                 The type  Type must be such that an object of type InputIt can be
                 dereferenced and then implicitly converted to  Type.


.SH Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

.SH Return value

   1) f
   2) \fI(none)\fP

.SH Complexity

   Exactly std::distance(first, last) applications of f.

.SH Exceptions

   The overload with a template parameter named ExecutionPolicy reports errors as
   follows:

     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

.SH Possible implementation

   See also the implementations in libstdc++, libc++ and MSVC stdlib.

   template<class InputIt, class UnaryFunc>
   constexpr UnaryFunc for_each(InputIt first, InputIt last, UnaryFunc f)
   {
       for (; first != last; ++first)
           f(*first);

       return f; // implicit move since C++11
   }

.SH Notes

   For overload \fB(1)\fP, f can be a stateful function object. The return value can be
   considered as the final state of the batch operation.

   For overload \fB(2)\fP, multiple copies of f may be created to perform parallel
   invocation. No value is returned because parallelization often does not permit
   efficient state accumulation.

.SH Example

   The following example uses a lambda-expression to increment all of the elements of a
   vector and then uses an overloaded operator() in a function object (a.k.a.,
   "functor") to compute their sum. Note that to compute the sum, it is recommended to
   use the dedicated algorithm std::accumulate.


// Run this code

 #include <algorithm>
 #include <iostream>
 #include <vector>

 int main()
 {
     std::vector<int> v{3, -4, 2, -8, 15, 267};

     auto print = [](const int& n) { std::cout << n << ' '; };

     std::cout << "before:\\t";
     std::for_each(v.cbegin(), v.cend(), print);
     std::cout << '\\n';

     // increment elements in-place
     std::for_each(v.begin(), v.end(), [](int &n) { n++; });

     std::cout << "after:\\t";
     std::for_each(v.cbegin(), v.cend(), print);
     std::cout << '\\n';

     struct Sum
     {
         void operator()(int n) { sum += n; }
         int sum {0};
     };

     // invoke Sum::operator() for each element
     Sum s = std::for_each(v.cbegin(), v.cend(), Sum());
     std::cout << "sum:\\t" << s.sum << '\\n';
 }

.SH Output:

 before: 3 -4 2 -8 15 267
 after:  4 -3 3 -7 16 268
 sum:    281

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to         Behavior as published              Correct behavior
                       it was unclear whether f can modify
                       the elements
   LWG 475  C++98      of the sequence being iterated over   made clear (allowed if the
                       (for_each is                          iterator type is mutable)
                       classified as “non-modifying sequence
                       operations”)
   LWG 2747 C++11      overload \fB(1)\fP returned std::move(f)    returns f (which
                                                             implicitly moves)

.SH See also

                         applies a function to a range of elements, storing results in
   transform             a destination range
                         \fI(function template)\fP
   for_each_n            applies a function object to the first N elements of a
   \fI(C++17)\fP               sequence
                         \fI(function template)\fP
   ranges::for_each      applies a function to a range of elements
   (C++20)               (niebloid)
   ranges::for_each_n    applies a function object to the first N elements of a
   (C++20)               sequence
                         (niebloid)
   range-for loop\fI(C++11)\fP executes loop over range
